An Engineer's Toolbox (2): Singleton and Debug Filtering

Singleton

Games almost always have shared data or services that multiple scripts need to read and update. A Singleton is one common way to centralize that access. In simple terms, it keeps one shared instance available through a static entry point.

Singleton tool

Singleton.cs

/**********************************************************
// Author   : K.(k79k06k02k)
// FileName : Singleton.cs
// Reference: http://wiki.unity3d.com/index.php/Singleton
**********************************************************/

public class Singleton<T> where T : class,new()
{
    private static T _instance;

    private static object _lock = new object();

    public static T Instance
    {
        get
        {

            lock (_lock)
            {
                if (_instance == null)
                {
                    _instance = new T();
                }

                return _instance;
            }
        }
    }
}

Implementation method: inherit from the generic class.

TestClass.cs

using UnityEngine;

public class TestClass : Singleton<TestClass>
{
    public void Hello()
    {
        Debug.Log("Hello");
    }
}

Call it like this:

TestClass.Instance.Hello();

Singleton tool (inherited from MonoBehaviour version)

SingletonObject.cs

/**********************************************************
// Author   : K.(k79k06k02k)
// FileName : SingletonObject.cs
// Reference: http://wiki.unity3d.com/index.php/Singleton
**********************************************************/
using UnityEngine;

public class SingletonObject<T> : MonoBehaviour where T : MonoBehaviour
{
	private static T _instance;

	private static object _lock = new object();

	public static T Instance
	{
		get
		{

			if (applicationIsQuitting) {
				Debug.LogWarning("[Singleton] Instance '"+ typeof(T) +
					"' already destroyed on application quit." +
					" Won't create again - returning null.");
				return null;
			}


            lock (_lock)
			{
				if (_instance == null)
				{
					_instance = (T) FindObjectOfType(typeof(T));


					if ( FindObjectsOfType(typeof(T)).Length > 1 )
					{
						Debug.LogError("[Singleton] Something went really wrong " +
							" - there should never be more than 1 singleton!" +
							" Reopenning the scene might fix it.");
						return _instance;
					}

					if (_instance == null)
					{
						GameObject singleton = new GameObject();
						_instance = singleton.AddComponent<T>();
						singleton.name = "(singleton) "+ typeof(T).ToString();


						DontDestroyOnLoad(singleton);

						Debug.Log("[Singleton] An instance of " + typeof(T) +
							" is needed in the scene, so '" + singleton +
							"' was created with DontDestroyOnLoad.");
					} else {
						Debug.Log("[Singleton] Using instance already created: " +
							_instance.gameObject.name);
					}
				}

				return _instance;
			}
		}
	}

	private static bool applicationIsQuitting = false;

	public void OnDestroy () {
		applicationIsQuitting = true;
	}
}

When to use it

For pure data or service access, such as Resources loading or game table data, use Singleton.cs. It does not need MonoBehaviour, so it avoids unnecessary component overhead.

Debug Filter

During development, Debug.Log ends up everywhere. Removing or disabling those calls one by one before release is a waste of time, so it is better to control them through code.

TestClass.cs

public class TestClass
{
    public static void Log(object message)
    {
#if Debug
        Debug.Log(message);
#endif
    }
}

Usage

Call this method anywhere you want to print a debug message.
Add the Debug symbol under File -> Build Settings -> Player Settings -> Other Settings -> Scripting Define Symbols.

Problem

This does remove logs from release builds, but it introduces another annoyance. When you double-click a Unity Console message, Unity jumps to the centralized wrapper method instead of the real call site. You then have to inspect the stack trace to find the useful line.

Improvement

One workaround is to define your own Debug class and use Conditional attributes. The example below follows the approach from Matt’s God Worship Record: Turn off Unity3D Debug.Log.

DebugFilterTools

DebugFilter.cs

/**********************************************************
// Author   : K.(k79k06k02k)
// FileName : DebugFilter.cs
// Reference: http://liangyenchen.blogspot.tw/2015/09/unity3d-debuglog.html
**********************************************************/
#if DEBUG_FILTER

using System.Diagnostics;
using UnityEngine;

public static class Debug
{
    [Conditional("DEBUG_FILTER")]
    public static void Break() { }
    [Conditional("DEBUG_FILTER")]
    public static void ClearDeveloperConsole() { }
    [Conditional("DEBUG_FILTER")]
    public static void DebugBreak() { }

    [Conditional("DEBUG_FILTER")]
    public static void DrawLine(Vector3 start, Vector3 end) { }
    [Conditional("DEBUG_FILTER")]
    public static void DrawLine(Vector3 start, Vector3 end, Color color) { }
    [Conditional("DEBUG_FILTER")]
    public static void DrawLine(Vector3 start, Vector3 end, Color color, float duration) { }
    [Conditional("DEBUG_FILTER")]
    public static void DrawLine(Vector3 start, Vector3 end, Color color, float duration, bool depthTest) { }

    [Conditional("DEBUG_FILTER")]
    public static void DrawRay(Vector3 start, Vector3 dir) { }
    [Conditional("DEBUG_FILTER")]
    public static void DrawRay(Vector3 start, Vector3 dir, Color color) { }
    [Conditional("DEBUG_FILTER")]
    public static void DrawRay(Vector3 start, Vector3 dir, Color color, float duration) { }
    [Conditional("DEBUG_FILTER")]
    public static void DrawRay(Vector3 start, Vector3 dir, Color color, float duration, bool depthTest) { }

    [Conditional("DEBUG_FILTER")]
    public static void Log(object message) { }
    [Conditional("DEBUG_FILTER")]
    public static void Log(object message, Object context) { }
    [Conditional("DEBUG_FILTER")]
    public static void LogFormat(string format, params object[] args) { }
    [Conditional("DEBUG_FILTER")]
    public static void LogFormat(Object context, string format, params object[] args) { }

    [Conditional("DEBUG_FILTER")]
    public static void LogWarning(object message) { }
    [Conditional("DEBUG_FILTER")]
    public static void LogWarning(object message, Object context) { }
    [Conditional("DEBUG_FILTER")]
    public static void LogWarningFormat(string format, params object[] args) { }
    [Conditional("DEBUG_FILTER")]
    public static void LogWarningFormat(Object context, string format, params object[] args) { }

    [Conditional("DEBUG_FILTER")]
    public static void LogError(object message) { }
    [Conditional("DEBUG_FILTER")]
    public static void LogError(object message, Object context) { }
    [Conditional("DEBUG_FILTER")]
    public static void LogErrorFormat(string format, params object[] args) { }
    [Conditional("DEBUG_FILTER")]
    public static void LogErrorFormat(Object context, string format, params object[] args) { }

    [Conditional("DEBUG_FILTER")]
    public static void LogException(System.Exception exception) { }
    [Conditional("DEBUG_FILTER")]
    public static void LogException(System.Exception exception, Object context) { }
}
#endif

Usage

Add the DEBUG_FILTER symbol under File -> Build Settings -> Player Settings -> Other Settings -> Scripting Define Symbols. Without the symbol, the debug calls are excluded.

Postscript

These Singleton and DebugFilter helpers are small, but they remove repeated friction during development. The same pattern can be adapted to your own production needs.

References

Attribution

Please credit ARKAI Studio and link back to this article when quoting or reposting.

This note covers two small but useful Unity helpers: a Singleton base class and a Debug filter. Singleton gives scripts a simple way to share data or services. Debug filtering keeps development logs from leaking into release builds, where they add noise and cost performance.

Other articles in this series

Singleton

Singleton tool

Singleton tool (inherited from MonoBehaviour version)

When to use it

Debug Filter

Usage

Problem

Improvement

DebugFilterTools

Usage

Postscript

References

Attribution